Cement manufacture, a complex capital intensive process, is currently conducted in kilns, commonly using coal as fuel. The process starts with mixing the raw materials, usually limestone or chalk, as a source of calcium carbonate, and clay or shale, as a source of aluminum silicates, with small quantities of other minerals added in the desired portions. The batch materials are crushed to the order of 200 mesh and mixed in a fluidization chamber for one or two days. The conventional technique of pyroprocessing cement clinker is to heat the mixed batch materials in a suspension preheater or precalciner In the process, calcium carbonate calcines to calcium oxide, and the exit temperature of the batch materials from the precalciner is approximately 900.degree. C. The batch materials then go to the kiln in which they are slowly heated to the clinkering temperature of approximately 1300.degree. C., at which temperature the clinker forms. The clinker, which is semi-melted and agglomerated or fused into rocks of several centimeters in size during the exothermic clinkering reaction, is then cooled in a clinker cooler. The clinker is then mixed with gypsum, fused to control final cement setting time, and crushed to about 325 mesh to form a powdered cement composition.
In the past, cement plants were principally natural gas fired. However, over the last 30 years or so many producers have used coal as a fuel because of its lower cost and because the ash and sulfur dioxide from coal burning can enter the cement batch materials without causing any damage to the product quality. For this reason coal burning does not cause much pollution. The only problems caused by coal burning are maintenance problems and the formation of alkali sulfates which are unacceptable in the clinker. To maintain alkali sulfate concentration within acceptable limits, part of the hot gas is bypassed at a location where the alkali sulfates are in vapor phase.
Reference is made to U.S. Pat. No. 3,469,828 for its disclosure of the use of inclined rotary kiln furnaces for calcining and clinkering of cement, and for modifying conventional rotary kiln furnaces in order to overcome some of the problems resulting therefrom. However inclined rotary kiln furnaces have inherent disadvantages for cement manufacture. The heat introduced to the rotary kiln produces an uneven temperature in the rotating kiln since the heat produces a higher temperature at the upper wall surface as the particulates travel down the lower wall surface of the rotating kiln. This can result in burning of the particulates as the overheated wall surface rotates into contact with the particulates. Even if the temperature is closely controlled to prevent burning, a large percentage of cement particles melt during the clinkering stage of passage through the rotary kiln since clinkering is an exothermic reaction. The temperature in the kiln must be high enough to initiate clinkering, and cannot be sufficiently reduced in the rotary kiln to prevent melting, agglomeration, wall-adhesion and other related problems.
References is also made to U.S. Pat. No. 3,692,285 for its disclosure of a vertical calcining furnace for drying and hardening iron ore pellets which are fed therethrough from a packed bed supply hopper in a zig-zag direction over the surfaces of ceramic balls and through preheating, indurating and cooling zones. The particles flow as a packed mass supported by the ceramic balls, and cooling air is directed upwardly through the beds of ceramic balls to cool the iron ore pellets prior to discharge. Such an apparatus is not suitable for clinkering cement since the particles are supported and packed together during heating, which will clog the apparatus if the particles are melted or agglomerated. Also the particles are unevenly heated and unevenly cooled in such an apparatus, which is unsatisfactory in the case of good quality cement compositions.
According to another prior-known process, disclosed in U.S. Pat. No. 4,002,422, a vertical shaft furnace is used for the heat reduction of particulate iron oxide material which is fed into the top thereof from a packed bed. Hot processing gas is introduced at an intermediate location to heat the bed of particulate material in an upper heat treatment zone as the particles fall therethrough to reduce the iron oxide to metallic iron. Cold processing gas is introduced radially-downwardly at an intermediate buffer zone to become heated by the iron particles falling therethrough for heat recovery purposes, the heated "cold" processing gas flowing upwardly to heat the incoming iron oxide particles in the same manner as the hot processing gas. Finally the falling iron particles pass through a lower converging cooling zone and exit onto an exterior conveyor. Such an apparatus is unsatisfactory for the preparation of cement clinkers since cement particles will melt and fuse or agglomerate if they are present in a packed bed at the top of a vertical shaft furnace; the exothermic clinkering reaction will be completed and will cause melting and agglomeration of the cement particles in the packed bed and/or in the upper heating zone of the furnace, and cooling of the agglomerates in the buffer zone or lower cooling zone will not restore the particles to their original dimensions. Also the particles are permitted to flow freely, under the effects of gravity, so that there is no way to adjust the residence time of the particles within zones of the furnace which may have different temperatures.
Reference is made to U.S. Pat. Nos. 4,584,022 and 4,595,416 which disclose fluidized bed cement clinkering methods and furnaces including a reactor furnace having a fluidized bed for receiving and clinkering therein a supply of sized pellets of feed material comprising cement-forming raw materials and carbon fuel. The feed material is combusted in the fluidized bed by additional fuel injected thereinto in the presence of fluidizing air forced up through a permeable support. Clinkered cement particles are discharged by gravity from the top of the fluidized bed into an overflow conduit to an outboard cooling tank containing a heat-exchanger for indirect cooling of the clinkers.
In the fluidized bed reactors of these patents the feed material is not suspended or heated substantially prior to entering the fluidized bed so that little or no calcining occurs outside the bed. Also the cooling of the clinkers occurs outside the furnace and the heat is not recovered for use in the calcining and clinkering reactions. Finally, the requirement of an outboard clinker-cooling tank increases the cost and size of the equipment.
Finally, reference is made to commonly-assigned U.S. Pat. No. 4,975,046 which discloses a furnace and processes for producing discrete clinkered cement pellets in which the batch materials are calcined, partially clinkered, and collected on a porous receiving base for completion of the clinkering reaction thereon while forcing cooling air upwardly therethrough to prevent agglomeration. The clinkered pellets are withdrawn from the bottom of the porous base at a rate which permits the pellets to complete clinkering between the time they enter the top of the bed and are withdrawn from the bottom thereof.